This invention relates to a polarization diversity receiver for use in optical heterodyne detection communication.
Among various optical communication systems, an optical heterodyne detection system or coherent optical fiber transmission system is excellent in view of its high reception sensitivity. This fact is described in an article contributed by Y. Yamamoto and T. Kimura to the IEEE Journal of Quantum Electronics, Volume QE-17, No. 6 (June 1981), pages 919 to 935, under the title of "Coherent Optical Fiber Transmission Systems".
In an optical heterodyne receiver, a local oscillation beam is coupled to an incident optical beam supplied to the receiver through an optical fiber. In order to achieve the high reception sensitivity, the incident optical and the local oscillation beams must have a common polarization state. It should be noted in this connection that the polarization state of the incident optical beam is subject to an irregular fluctuation due to disturbances or turbulances which are inevitably caused on the optical beam in the optical fiber. The optical heterodyne receiver must therefore comprise a circuit which compensates for the irregular fluctuation in the polarization state of the incident optical beam.
For use as the optical heterodyne receiver, a polarization diversity receiver is desirable in compensating for the irregular fluctuation in the polarization state of the incident optical beam. In the manner known in the art, the polarization diversity receiver comprises a directional optical coupler for coupling the incident optical beam and the local oscillation beam into a coupled optical beam. A polarization beam splitter is used in splitting the coupled optical beam into first and second optical beams having planes of polarization which are orthogonal to each other. First and second optical detectors or photodetectors are used to convert the first and the second optical beams into first and second intermediate frequency signals.
In the polarization diversity receiver, a processor is used to carry out combination and demodulation of the first and the second intermediate frequency signals to produce a receiver output signal. In each of the first and the second optical beams, the incident optical and the local oscillation beams have components, each of which is a plane or linearly polarized beam and which have a common plane of polarization. No signal loss therefore appears due to incoincidence in the polarization state between the incident optical and the local oscillation beams.
The processor may first demodulate the first and the second intermediate frequency signals into first and second baseband signals and thereafter combine the first and the second baseband signals into the receiver output signal. When the processor is operable in this manner, the polarization diversity receiver will be said to be of a baseband combining type.
In the polarization diversity receiver of the baseband combining type, a supplying circuit is used in supplying the first and the second intermediate frequency signals to first and second demodulator circuits. According to prior art, the supplying circuit may consist of mere connections between the first and the second optical detectors and the first and the second demodulator circuits.
Each of the demodulator circuits is for carrying out square-law detection on a pertinent one of the first and the second intermediate frequency signals that is supplied to the demodulator circuit under consideration. A signal combiner is used in combining the first and the second baseband signals into the receiver output signal. The square-law detection may be either delay detection or square-law envelope detection.
In an article contributed by B. Glance to the Journal of Lightwave Technology, Volume LT-5, No. 2 (February 1987), pages 274 to 276, under the title of "Polarization Independent Coherent Optical Receiver", a bit error rate is calculated in connection with a polarization diversity receiver of the baseband combining type, in which differential phase shift keying (DPSK) is used as the delay detection. As a result of computer simulation, Glance shows that the polarization diversity deteriorates or degrades the reception sensitivity only 0.4 dB irrespective of the polarization state of the incident optical beam.
It should be noted that the above-described result of Glance is applicable to a polarization diversity receiver which is of the baseband combining type and comprises two ideal demodulator circuits. More particularly, each demodulator circuit should have a very wide dynamic range in order to keep the reception sensitivity excellent irrespective of the polarization state of the incident optical beam. It is, however, very difficult to realize a wide-dynamic-range demodulator circuit which is moreover operable in a broad frequency band used in the optical communication.
In practice, each of the demodulator circuits is operable in a restricted or limited dynamic range. This results in a sensitivity deterioration or degradation. In fact, it is described in a report contributed by B. Enning et al to the OFC '88 Technical Digest as Paper No. TU15 and under the title of "Polarization-diversity Receiver for 560-Mbit/s ASK Heterodyne Detection" that the sensitivity deterioration amounts to 1 dB in a polarization diversity receiver which is of the baseband combining type and comprises demodulator circuits having a restricted dynamic range.
Not only to the restricted dynamic range of the demodulator circuits, attention must be directed but also to the fact that the demodulator circuits comprise circuit elements having characteristics which may not necessarily be uniform. When the characteristics of a pair of corresponding circuit elements are not identical with each other in the two demodulator circuits, a fluctuation is unavoidable in the sensitivity deterioration. This adversely affects the reception sensitivity.
Incidentally, it may be mentioned here that frequency shift keying (FSK) single filter envelope detection is described in a letter contributed by K. Emura et al to the Electronics Letters, Volume 20, No. 24 (the 22nd Feb. 1984), pages 1022 and 1023, under the title of "Novel Optical FSK Heterodyne Single Filter Detection System Using a Directly Modulated DFB-Laser Diodes". Differential phase shift keying detection is discussed in a report contributed by K. Emura et al to the IOOC-ECOC '85 Technical Digest, pages 401 to 403, under the title of "400 Mb/s Optical DPSK Heterodyne Detection Experiments Using DBR Laser Diodes with External Optical Feedback".
It may furthermore be mentioned here that a balanced receiver structure can be used in place of a combination of the directional optical coupler, the polarization beam splitter, and the first and the second optical detectors. The balanced receiver structure is disclosed by the present inventor et al in U.S. patent application Ser. No. 291,885 filed Dec. 29, 1988 (European Patent Application No. 88 12 1791.3 filed the 28th Dec. 1988).
In the balanced receiver structure, the incident optical beam alone is split into first and second signal beams having orthogonal planes of polarization. The local oscillation beam is divided into first and second local beams. Two directional optical couplers are used in coupling the first signal beam and the first local beam into the first optical beam and the second signal beam and the second local beam into the second optical beam. Each of the optical detectors includes two optical detectors whose output signals are differentially combined into one of the first and the second intermediate frequency signals.
It is therefore possible to understand that a polarization diversity receiver comprises (1) a coupling and splitting arrangement for coupling and splitting an incident optical beam and a local oscillation beam to produce first and second optical beams including incident optical beam components which have orthogonal planes of polarization and (2) an optical detecting arrangement for detecting the first and the second optical beams to produce first and second intermediate frequency signals.